1. Field of the Invention
The present invention relates to an ion attachment mass spectrometry method, more particularly, relates to an ion attachment mass spectrometry method for quantitative analysis enabling accurate measurement of the concentration of a measured gas, and apparatus used therefor.
2. Description of the Related Art
Ion attachment mass spectrometry (IAMS) is a method of ionizing the molecules of a measured gas without causing fragmentation, making the ions of the molecules move to the mass spectrometry region, and analyzing their mass there.
Examples of apparatuses related to ion attachment mass spectrometry are disclosed in JP-A-6-11485, JP-A-2001-174437, JP-A-2001-351567, JP-A-2001-351568, JP-A-2002-124208, and JP-A-2002-170518. Further, as other documents, (1) Hodge, “Analytical Chemistry”, 1976, vol. 48, no. 6, p. 825, (2) Bombick, “Analytical Chemistry”, 1984, vol. 56, no. 3, p. 396, (3) Fujii, “Analytical Chemistry”, 1986, vol. 61, no. 9, p. 1026, (4) Fujii, “Chemical Physics Letters”, 1992, vol. 191, no. 1.2, p. 162, and (5) Fujii, “Rapid Communication in Mass Spectrometry”, 2000, vol. 14, p. 1066 may be mentioned.
Referring to FIG. 6, the general configuration of an apparatus for ion attachment mass spectrometry will be explained. In FIG. 6, 1 indicates a metal ion generation region, 2 an attachment region, and 3 a mass spectrometry region. A differential exhaust region 4 is provided between the attachment region 2 and mass spectrometry region 3. The metal ion generation region 1 is provided with an emitter 11 and a repeller 12. Further, 21 indicates a mechanism for introduction of measured gas or sample gas, while 22 indicates a mechanism for introduction of an adjustment gas. The measured gas and the adjustment gas are introduced into the attachment region 2. Partitions 23 and 41 respectively having apertures 23a and 41a at their centers are provided between the attachment region 2 and differential exhaust region 4 and between the differential exhaust region 4 and the vacuum analysis region 3 respectively. A container part forming the attachment region is provided with a vacuum pump 24 and pressure gauge 25. The container part forming the mass spectrometry region is provided with a mass spectrometer 31 and vacuum pump 32. Further, the differential exhaust region 4 is provided with an exclusive vacuum pump 42.
In the ion attachment mass spectrometry apparatus of the above related art, all of the metal ion generation region 1, the attachment region 2, the mass spectrometry region 3, and the differential exhaust region 4 are at reduced pressure of not more than atmospheric pressure. In the metal ion generation region 1, an emitter 11 of an oxide of an alkali metal is heated to generate Li+ and other positively charged metal ions. The metal ions are transported to the attachment region 2 from the metal ion generation region 1 by the repulsion force of the repeller 12.
The measured gas is introduced into the attachment region 2. In the attachment region 2, the metal ions gently attach to locations with a concentration of charges of molecules of the measured gas. The molecules to which the metal ions are attached become positively charged ions as a whole, whereby attached ions are generated. At the time of attachment, the surplus energy is extremely small, so disassociation does not occur.
However, to stabilize the attached ions so that the metal ions do not again disassociate from the molecules, it is necessary to strip the surplus energy by having the ions strike the ambient gas. To secure the maximum efficiency of stripping of the surplus energy, the pressure of the attachment region 2 of the related art is made about 100 Pa. The measured gas can be a gas for stripping the surplus energy, but usually the low reactivity N2 gas etc. is separately introduced as the adjustment gas.
The adjustment gas also has another important role of decelerating the metal ions. To efficiently emit the metal ions from the emitter 11 and transport them to the attachment region 2, a translational energy of at least 10 eV is required in the metal ions. If the translational energy is high, even if the metal ions contact the molecules, they will not attach to the molecules, but will end up being reflected and running away without being ionized. Therefore, the metal ions emitted and transported with the high translational energy are made to strike the ambient gas at the attachment region 2 a large number of times to make them decelerate. For sufficient deceleration, the pressure of the attachment region 2 should be about 100 Pa.
The attached ions are transported to the mass spectrometry region 3 and are separated in mass and the intensity is measured by a Q-pole type mass spectrometer or other mass spectrometer 31 using electromagnetic force. The mass spectrometer 31 can only operate under a pressure of not more than 10−3 Pa, so the partitions 23 and 41 are provided between the attachment region 2 and mass spectrometry region 3 for creating a pressure difference.
FIG. 6 shows a general example of the related art. In an actual example of the related art, there is a partial difference from the above-mentioned related art of FIG. 6 due to the presence or absence of the differential exhaust region or vacuum pump. In addition, in each example of the related art, there are the following characteristic differences. In the document (1), the attached ions are made intermittently (pulses) by the fast changing electric field and the signal detected by a lock-in amplifier. Further, the adjustment gas containing a slight amount of the measured gas is introduced into the attachment region and the pressure made not more than 6 Pa. For this adjustment gas, the reaction gas is used and made to react once with metal ions, then the metal ions are shifted from the reaction product to the measured gas. In the document (2), K+ is used as the metal ions and the pressure is made at least 2.4 Pa. In the document (5), the adjustment gas (He gas) of 0.1 Pa and the measured gas of 5×10−3 are introduced into the attachment region. However, since an ion trap mass spectrometer using an internal ionization system is employed, the attachment region and mass spectrometry region are in the same region, the metal ions attach to the measured gas in the process of vibration due to the high frequency field, and the attached ions become unstable in vibration and are emitted to the detector.
According to the conventional ion mass spectrometry method, in each case, ionization is possible without causing disassociation of the molecules of the measured gas, and the ingredients of the measured gas (qualitative analysis) can be measured by a high accuracy. This surpasses other analytical methods of the ionization system. Much is expected for the ion attachment mass spectrometry method from the scientific and industrial viewpoints.
However, according to the ion attachment mass spectrometry method, there are the following three problems (1) to (3) directly relating to the measurement of the concentration of ingredients (quantitative analysis).
(1) The stability and reproducibility of the signal are poor. (2) The linearity is poor. That is, an accurate proportional relationship cannot be established between the concentration of the measured gas and signal intensity. (3) The measured gas cannot be measured in a state of only the measured gas, that is, without the introduction of the adjustment gas.
Further, the ion attachment mass spectrometry method suffers from the following three problems (4) to (6) relating indirectly to the measurement of concentration of the ingredients (quantitative analysis).
(4) The apparatus is large in size and quality change of the measured gas in the apparatus may be possible. (5) A plurality of vacuum pumps is required and stable evacuation of the measured gas is difficult to secure. (6) Light and volatile matter enter the mass spectrometry region from the emitter and inhibit stable measurement.